Stoichiometry is a central part of every Chemistry course. At the heart of the topic, the fundamental basis of stoichiometry, is the idea that atoms, and therefore mass, are conserved in a chemical reaction. Yet far too often, this foundational piece is glossed over in a rush to use dimensional analysis and conversion factors to determine (as an example) the product mass from the reactant mass. This Concept Builder is an effort to slow down the march towards conversions factors and converting and to give students a moment to think about mass conservation.

In the activity, students are given a balanced chemical equation. From the equation they determine the number of atoms of each element and enter the values into a blank for the reactant side and the product side. Then they use molar mass values to determine the mass of each element on the reactant and the product side. There are two conceptual questions accompanying every problem - a question about atoms being conserved and a question about the mass of each element being conserved. The exercise provides a great way to emphasize the nature of a chemical reaction as the rearrangement of atoms. 

The Concept Builder consists of three difficulty levels - Apprentice, Master, and Wizard levels. In the Apprentice level, there is a total of two elements on the reactant and product side (typically a synthesis reaction). In the Master level, there is a total of three elements in the reactant and product species. And finally, the Wizard level includes reactions with four elements.

The Concept Builder is intended to be used relatively early in a unit on stoichiometry. Ideally, it would be incorporated into a lesson shortly after balancing chemical equations and using molar mass values to convert from moles to mass. Students will need to be able to count atoms and be able to use periodic table molar mass values to determine the mass from the moles. Calculations must be within 2% of the keyed answer to be regarded as correct. We recommend the use of three or more significant digits.

This Concept Builder was intended as an in-class activity. Teachers using the Concept Builder with their classes should preview the activity (or view the Questions in a separate file) in order to judge which levels would be most appropriate for their students.

 

The most valuable (and most overlooked) aspect of this Concept Builder is the Help Me! feature. Each question group is accompanied by a Help page that discusses the specifics of the question. This Help feature transforms the activity from a question-answering activity into a concept-building activity. The student who takes the time to use the Help pages can be transformed from a guesser to a learner and from an unsure student to a confident student. The "meat and potatoes" of the Help pages are in the sections titled "How to Think About This Situation:" Students need to be encouraged by teachers to use the Help Me! button and to read this section of the page. A student that takes time to reflect upon how they are answering the question and how an expert would think about the situation can transform their naivete into expertise. 

 

 

Visit: Concept Builder | Questions (for Task Tracker Teachers only)